1. Field of the Invention
The current invention relates to location of a device. More particularly, the present invention relates to a method and apparatus for determining the location of a device using a coarse estimate of the position of the device and estimates of the location of transmitters used to determine that coarse estimate.
2. Description of the Related Art
A common means by which to locate a device is to determine the amount of time required for signals transmitted from multiple sources at known locations to reach a receiver within the device to be located. One system that provides signals from a plurality of transmitters of known location is the well-known Global Positioning Satellite (GPS) system. The position of the satellites within the GPS system can be identified by a number of different pieces of information, some more accurate than others. For example, a reasonably accurate determination of the location of a GPS satellite can be made if the time of day at which a set of information, known as the “Ephemeris”, is available. Transmissions from each satellite provide the time of day and the Ephemeris for the particular transmitting satellite. The Ephemeris provides information defining the orbits of the particular satellites from which the Ephemeris was received.
In addition, all of the satellites transmit another set of information, referred to as “Almanac”. The Almanac includes less accurate information regarding the location of all of the satellites in the “constellation”. Both the Almanac and the Ephemeris information is valid for a limited amount of time. The Almanac information is considered to be accurate to approximately 3 kilometers for approximately one week from the time the Almanac is transmitted. The Ephemeris provides information regarding the satellite orbits with an accuracy of approximately 1 meter for a period of time equal to approximately 2 hours. The errors in both the Almanac information and the Ephemeris information grow as the data ages. Accordingly, information regarding the location of the satellites is less and less accurate as the Almanac and Ephemeris age, unless updated information is received at appropriate intervals in time.
Without accurate information regarding the location of the satellites, the location that is determined based on the receipt of signals transmitted from the satellites will be inaccurate. Therefore, it is necessary to receive updates from the satellite or alternatively from an alternative source. One such alternative source is a wireless communication system base station that has a GPS receiver capable of attaining the required information from the GPS satellites. However, for the device to be located to attain the information at regular intervals consumes valuable resources, such as power required to receive the information, and bandwidth required to transmit the information from a remote source to the device. Accordingly, there is currently a need for a means by which accurate determinations of the position of a receiver can be made with a minimal expenditure of resources. This need is particularly acute when using systems in which transmitters move over time and the location of such transmitters is known accurately only upon receiving updates from a source remote to the device performing the location calculation. The method and apparatus disclosed herein satisfies this need.